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United States Patent |
5,561,524
|
Yamasaki
,   et al.
|
October 1, 1996
|
Interferometric distance measuring apparatus utilizing an
asymmetric/elliptic beam
Abstract
An interferometric distance measuring interferometer comprises a light
source (11) for irradiating a main light beam (L), a beam splitting member
(56) for splitting the main light beam into a first light beam (MR1, MR2)
and a second light beam (MM1, MM2), a first reflection member (28) and
second reflection member (14) for reflecting the first light beam and the
second light beam, respectively, an optical composition member (25) for
compositing the first light beam reflected by the first reflection member
(28) and the second light beam reflected by the second reflection member
(14), and a photoelectric converter (19) for receiving the composited
light beam from the optical composition member. Whereby, a relative
displacement between the first reflection member (28) and the second
reflection member (14) is detected based on a photoelectric conversion
signal from the photoelectric converter (19). The apparatus according to
the present invention further comprises an asymmetric conversion member
(12) for establishing the optical intensity distribution of a
cross-section of at least one beam selected from a group of light beams
including the main light beam (L) irradiated from the light source (11),
the first light beam (MR1, MR2) and the second light beam (MM1, MM2). The
asymmetric conversion member is arranged at at least one portion on an
optical path of the light beam between the light source (11) and the
photoelectric converter (19).
Inventors:
|
Yamasaki; Shigeru (Saitama-ken, JP);
Sueyoshi; Masafumi (Kanagawa-ken, JP)
|
Assignee:
|
Nikon Corporation (Tokyo, JP)
|
Appl. No.:
|
364193 |
Filed:
|
December 27, 1994 |
Foreign Application Priority Data
| Dec 27, 1993[JP] | 5-331191 |
| Jul 20, 1994[JP] | 6-167831 |
| Nov 08, 1994[JP] | 6-273424 |
Current U.S. Class: |
356/493; 356/500 |
Intern'l Class: |
G01B 009/02 |
Field of Search: |
356/345,351,358,349,363
|
References Cited
U.S. Patent Documents
4650330 | Mar., 1987 | Fujita | 356/349.
|
Foreign Patent Documents |
62-150721 | Jul., 1987 | JP.
| |
Primary Examiner: Turner; Samuel A.
Assistant Examiner: Kim; Robert
Attorney, Agent or Firm: Armstrong, Westerman, Hattori, McLeland & Naughton
Claims
What is claimed is:
1. An interferometric distance measuring apparatus comprising
a light source for irradiating a main light beam;
a beam splitting member for splitting said main light beam into a first
light beam and a second light beam;
a first reflection member and second reflection member for reflecting said
first light beam and said second light beam, respectively;
an optical composition member for compositing said first light beam
reflected by said first reflection member and said second light beam
reflected by said second reflection member; and
a photoelectric converter for receiving the composited light beam from said
optical composition member;
whereby a relative displacement between said first reflection member and
said second reflection member is detected based on a photoelectric
conversion signal from said photoelectric converter,
characterized in that said apparatus further comprises an asymmetric
conversion member arranged on at least a portion of an optical path
between the light source and the photoelectric converter,
said asymmetric conversion member converting at least one beam, which is
selected from a group of light beams including the main light beam
irradiated from said light source, said first light beam and said second
light beam, into a parallel luminous flux, and
said asymmetric conversion member further converting a cross-sectional
shape of said at least one beam into an asymmetric cross-sectional shape.
2. An apparatus as defined in claim 1, characterized in that said
asymmetric conversion member is at least one anamorphic prism arranged on
said optical path between said light source and said photoelectric
converter.
3. An apparatus as defined in claim 1, Characterized in that said
asymmetric conversion member is a pair 0f cylindrical lenses arranged on
said optical path between said light source and said photoelectric
converter.
4. An apparatus as defined in claim 1, characterized in that said
asymmetric conversion member is arranged between said light source and
said beam splitting member.
5. An apparatus as defined in claim 1, characterized in that, as said light
source, an asymmetric light source for radiating said main light beam on
condition that the cross-sectional Shape of the beam is asymmetric is
used, and that said asymmetric light source therefore also serves as said
asymmetric conversion member.
6. An apparatus as defined in claim 1, characterized in that said
asymmetric cross-sectional shape of the beam is an ellipse, in which a
ratio of a length thereof to a breadth thereof is about 2.
7. An apparatus as defined in claim 1, further comprising a third
reflection member used for a double path type interferometric distance
measuring apparatus, said third reflection member reflecting the first
light beam reflected by the first reflection member as well as the second
light beam reflected by the second reflection member to said optical
composition member.
8. An apparatus as defined in claim 7, characterized in that said third
reflection member is a corner-cube.
9. An interferometric distance measuring apparatus comprising:
a light source;
a beam splitter for splitting a laser beam irradiated from said light
source into a first beam and a second beam;
a first mirror and a second mirror for reflecting said first beam and said
second beam, respectively;
an optical composition member for compositing said first beam reflected by
said first mirror and said second beam reflected by said second mirror;
a photoelectric detector for receiving the composited beam from said
optical composition member;
a table carrying said second mirror and movable to a desired position: and
an asymmetric conversion member converting at least one beam, which is
selected from a group of light beams including the main light beam
irradiated from said light source, said first light beam and said second
light beam into a parallel luminous flux,
said asymmetric conversion member further converting a cross-sectional
shape of said at least one beam into an asymmetric cross-sectional shape,
a direction of the asymmetric conversion of the cross-section of said at
least one beam being dependent on a direction of a tilt of the table
together with the second mirror so as to compensate for a displacement of
the second beam reflected by the second mirror said displacement being
caused by said tilt of the table.
10. An apparatus as defined in claim 9, wherein said asymmetric conversion
member is at least one anamorphic prism or a pair of cylindrical lenses
arranged between said light source and said photoelectric detector.
11. An apparatus as defined in claim 9, characterized in that said
asymmetric cross-sectional shape of the beam is an ellipse, in which a
ratio of a length thereof to a breadth thereof is about 2.
12. An interferometric distance measuring apparatus comprising:
a table movable to a desired position;
a first mirror;
a second mirror different from said first mirror and mounted on said table;
a light emitting system for projecting a laser beam to said first mirror
and to said second mirror, respectively;
a light receiving system for receiving and compositing first and second
laser beams reflected by said first and second mirrors, respectively; and
a laser beam cross-section shaping member for shaping a cross-section of
said laser beam into an asymmetric cross-sectional shape, such that a
direction of shaping the cross-section of said laser beam is dependent on
a direction of a tilt of the table together with the second mirror So as
to compensate for a displacement of the second beam reflected by the
second mirror, said displacement being caused by said tilt of the table.
13. An apparatus as defined in claim 12, characterized in that said
asymmetric cross-sectional shape of the beam is an ellipse, in which a
ratio of a length thereof to a breadth thereof is about 2.
Description
BACKGROUND OF THE INVENTION
The present invention relates to an interferometric distance measuring
apparatus so-called, an interferometer such as a laser interferometer
which is used for measuring with high precision an amount of movement etc.
of a stage device of an exposure apparatus for producing semiconductors or
of a precision measuring instrument.
PRIOR ART
In an exposure apparatus for producing semiconductors, for example, an
XY-stage is used in order to position a photosensitive substrate with high
precision, wherein a laser interferometer is provided in order to measure
the coordinate positions of the XY-stage with high precision. Further, in
a precision measuring instrument or in a superprecision machine tool, a
laser interferometer is provided in order to measure the coordinate
positions of the stage which positions an object to be measured.
FIG. 12 shows a stage device provided with a prior laser interferometer. In
FIG. 12, a laser beam L with a circular cross-sectional shape is
irradiated from a laser light source 1 such as a He--Ne laser light source
enters a polarization beam splitter 2. The laser beam L comprises two
polarized light components (P-polarized light and S-polarized light), the
direction of polarization of which are orthogonal to each other. A
polarized light component polarized in the direction perpendicular to an
incident surface, i.e. S-polarized light component, is reflected by the
polarized beam splitter 2 and directed as a reference beam LR to a fixed
mirror 4 through a 1/4-wave plate This interferometer can be adapted for
use with the heterodyne interference method or the homodyne interference
method, wherein in the former a slight difference between frequencies of
the two polarized light components is given and in the latter no
difference therebetween is given. The reference beam LR, after being
reflected by the reference mirror 4, passes through the 1/4-wave plate 3R,
and is transmitted as P-polarization light through the polarized beam
splitter 2, and thereafter enters a receiver 7 including an analyzer and a
photoelectric conversion element.
On the other hand, the other polarized light component polarized in the
direction parallel to the incident surface, i.e. P-polarized light
component, is transmitted through the polarization beam splitter 2 and
directed as a measurement beam LM through the 1/4-wave plate 3M to a
movable mirror 5. The movable mirror 5 is limed to one end of the stage 6
which is movable along the Y-axis parallel to the optical axis of the
measurement beam LM. The measurement beam LM reflected by the movable
mirror 5, after passing through the 1/4-wave plate 3M and then reflected
as S-polarized light by the polarization beam splitter 2, enters the
receiver 7 substantially coaxially with the reference beam LR. The
receiver 7 conducts the photoelectric conversion of an interference light
formed from the reference beam LR and the measurement beam LM, thereby
generating digital pulses which correspond to an amount of movement of the
movable mirror 5, e.g. in the Y-direction movement. By integrating these
digital pulses, the amount of movement of the movable mirror 5 along the
Y-axis can be measured.
In order to measure the amount of movement of the movable mirror 5 with a
laser interferometer as described above, an interference region where the
measurement beam LM and the reference beam LR are superposed must exist in
the light receiving portion of the receiver 7.
With reference to the above, when a movable portion of the stage 6, in
Other words, the movable mirror 5 has to be tilted during the measurement
of the position of the movable mirror 5 based on the reason that a
pitching or rolling adjustment, e.g. is needed for a table on which the
movable mirror 5 is mounted, the position of the reflected measurement
beam LM is displaced due to the tilting of the movable mirror 5, as shown
in FIG. 13. If an area of the interference region in the light-receiving
portion of the receiver 7 is reduced due to this positional displacement,
a signal intensity (S/N ratio) of the photoelectric conversion signal of
the interference light is also reduced. When the signal intensity falls
below a level of detection sensitivity of the photoelectric conversion
element, it is not possible to accurately measure the movement.
Furthermore, regardless of the size of a diameter of the measurement beam
LM, an amount of displacement of the measurement beam LM corresponding to
the predetermined tilting angle of the movable mirror 5 is constant.
Therefore, in order to reduce the occurrence of undesirable effects caused
by the tilting of the movable mirror 5, it is better to enlarge diameters
of the measurement beam LM and the reference beam LR. Consequently, in the
prior art, the beam diameter of the laser beam L irradiated from the laser
light source was enlarged, whereby the desired range (stroke of a tilting
angle) over which the movable mirror 5 is allowed to tilt could be
obtained.
FIG. 14 shows a stage of superposition of a circular reference beam LR and
a circular measurement beam LM in the light-receiving portion of the
receiver 7 (FIG. 13). If it is assumed that an intensity distribution of
each beam is uniform in FIG. 14, a signal intensity obtained by a
photoelectric conversion of the interference light is defined as I, said
interference light being obtained from the complete superposition of the
measurement beam LM and the reference beam LR. Assuming that a radius of
each Of the measurement beam LM and the reference beam LR is r, a signal
intensity I(t) obtained when the reference beam LR and the measurement
beam LM are displaced each other by the distance t is expressed by the
following formula:
I(t)=2 (.theta.-sin.theta. cos.theta.)/.pi.
with
.theta.=cos.sup.-1 (t/(2r)), 0.ltoreq..THETA..ltoreq..pi./2
It is understood from this formula that, by enlarging the beam diameter
(2r), the reduction of the signal intensity of the interference light can
be suppressed on condition that the amount of displacement t is maintained
constant. Even if an output of the laser light source 1 is maintained
constant regardless of the beam diameter, a permissible amount of
displacement of the measurement beam LM becomes larger as the beam radius
r becomes larger, on condition that detection sensitivity of the receiver
7 of the laser interferometer is maintained constant.
While the laser interferometer as shown in FIG. 12 is an example of a
single path type interferometer, it is difficult to actually use this
single path typo interfarometer under the condition that the movable
mirror 5 is tilted to a great extent, because, when the movable mirror 5
is tilted, inclination is caused between the wave plane of the measurement
beam LM and the wave plane of the reference beam LR so that the S/N ratio
of the interference signal drops drastically.
Therefore, a double path type interferometer is generally used, because
inclination does not arise between the wave planes of both beams even when
displacement occurs between the measurement beam and the reference beam as
a result of tilting of movable mirror. In the double path type
interferometer, the reference beam and the measurement beam travel
reciprocally twice between the polarization beam splitter and the fixed
mirror and between the polarization beam splitter and the movable mirror,
respectively, order to perform reciprocal travel a second time, a
corner-cube is used.
FIG. 15a shows a corner-cube 8 of diameter .phi.1 used in a double path
type laser interferometer. In this FIG. 15a, the reference beam LR1 and
the measurement beam LM1 which have already traveled reciprocally for the
first time enter the corner-cube 8. The two beams are then reflected by
the corner-cube 8 and directed to the polarization beam splitter as a
reference beam LR2 and a measurement beam LM2 performing reciprocal travel
the second time. The space between the beam performing reciprocal travel
the first time and the beam performing reciprocal travel the second time
is defined as W1.
PROBLEMS OF PRIOR ART
In the prior laser interferometer, undesirable effects caused by the
displacement of the measurement beam through the tilting of the movable
mirror can be reduced by enlarging the beam diameter of the laser beam.
However, in a system in which the beam diameter is enlarged, the size of
optical parts of the laser interferometer should also be enlarged.
When the optical parts are enlarged in such a way, however, the entire
construction of the interferometer is also enlarged, which results in a
difficulty in incorporating the interferometer into various stage devices.
Furthermore, in the double path system using a corner-cube 8 as shown in
FIG. 15a, if the space W1 between two light beams is large, it is required
to make the outer diameter of the corner-cube 8 larger. Therefore, in
order to make the size of optical parts smaller, it is preferable to make
the space W1 as small as possible. Further, in the double path system, if
the beam diameter is made larger without changing the beam space W1 in
order to reduce the occurrence of undesirable effects caused by the
displacement of the measurement beam due to the tilting Of the movable
mirror, as shown by the reference beams LR3 and LR4 in FIG. 15b, the beams
LR3 and LR4 would strike the ridge lines (lines dividing the reflecting
surface into three equal parts about a center of the optical axis) as well
as contour lines of the corner-cube 8.
Even when the beams strike the ridge lines in such a way, an ideal
corner-cube would be free from problems, but, in actuality, the ridge line
has a certain width. Furthermore, there may exist an error of the degree
of a right angle formed by two reflecting surfaces between which a ridge
line in question exists. Therefore, there is a possibility of an adverse
effect being exerted on a measurement operation as a result of turbulence
occurring in a wave plane of the laser beam emitted from the corner-cube.
Therefore, it is preferable to use a large corner-cube 9 having a diameter
.phi.2, as shown in FIG. 15c, whereby it is also desirable to establish a
beam space as W2, such that the reference beams LR3 and LR4 do not strike
the ridge lines and contour lines.
In this case, however, the beam space W2 is unfortunately made larger which
runs contrary to the desirable situation to reduce the size of optical
parts as described above. Namely, in this case, it is required to use a
corner-cube 9 having a large diameter, thereby enlarging the size of
optical parts. Other disadvantages include an increase in costs and
difficulty in mounting the interferometer into the stage devices etc.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an interferemetric
distance measuring apparatus, so-called, an interferometer intended to
establish the optical intensity distribution in a cross-section of a light
beam along the optical path so as to be asymmetric such that a shape of
the cross-section can be expanded in the predetermined direction (e.g.,
from a circular cross-sectional shape to an elliptic one), for making
permissible amount of displacement between the reference beam and the
measurement beam in said predetermined direction larger, without enlarging
the size of optical parts of the interferometer, thereby enabling the size
of said optical parts to be smaller.
It is another object of the present invention to provide an interferemetric
distance measuring apparatus intended to establish the optical intensity
distribution in a cross-section of a light beam so as to be asymmetric as
above-mentioned when it is applied to a double path type interferometer
with a corner-cube, for example, for making the beam space between each
path of double paths smaller than that in the case of a circular (i.e.,
symmetric) cross-section, under the condition that the same amount of
displacement occurs, thereby making the corner-cube smaller. In other
words, said permissible amount of displacement mentioned above can be made
larger than that in the prior art, under the condition that the size of
the corner-cube is maintained constant.
An interferemetric distance measuring apparatus according to the present
invention for achieving the above-mentioned objects, as shown in FIGS.
1-3, comprises a light source (11) for irradiating a main light beam (L),
a beam splitting member (26) for splitting the main light beam into a
first light beam (MR1, MR2) and a second light beam (MM1, MM2),
respectively, a first reflection member (28) and a second reflection
member (14) for reflecting the first light beam and the second light beam,
respectively, an optical composition member (26) for compositing the first
light beam reflected by the first reflection member (28) and the second
light beam reflected by the second reflection member (14), and a
photoelectric convertor (19) for receiving The composited light beam from
the optical composition member, whereby a relative displacement between
the first reflection member (28) and the second reflection member (14) is
detected based on a photoelectric conversion signal from the photoelectric
converter (19), said apparatus further comprising an asymmetric conversion
member (12) for establishing the optical intensity distribution of a
cross-section of at least one beam selected from a group of light beams
including %he main light beam (L) irradiated from said light source (11),
said first light beam (MR1, MR2) and said second light beam (MM1, MM2),
said asymmetric conversion member being arranged at least one portion on
an optical path of light beam between the light source (11) and the
photoelectric converter (19), said optical intensity distribution being
asymmetric relative to a center of said optical intensity distribution.
In this case, an example of the asymmetric conversion member is one or more
anamorphic prisms (24, 25) arranged on an optical path between the light
source (11) and the photoelectric converter (19). An anamorphic prism is
defined as a prism wherein two deflection angles in two different
directions on a cross-section of an entering light beam are different from
each other.
Another example of the asymmetric conversion member is a pair of
cylindrical lenses (51A, 51B) arranged on an optical path between the
light source (11) and the photoelectric converter (19).
In addition, in the case that a main light beam (L) with a symmetric
optical intensity distribution is irradiated from the light source (11),
it is preferable that the asymmetric conversion member is arranged between
the light source (11) and the beam splitting member
However, as the light source, an asymmetric light source (40, 41) for
irradiating said main light beam such that an optical intensity
distribution thereof is asymmetric can be used, whereby the asymmetric
light source also serves as an asymmetric conversion member.
Furthermore, an interferemetric distance measuring apparatus according to
the present invention comprises a light source (11) for irradiating a main
light beam (L), a beam splitting member (26) for splitting the main light
beam into a first light beam (MR1, MR2) and a second light beam (MM1,
MM2), a first reflection member (28) and a second reflection member (14)
for reflecting the first light beam and the second light beam,
respectively, an optical composition member (26) for compositing the first
light beam reflected by the first reflection member (28) and the second
light beam reflected by the second reflection member (14), and a
photoelectric converter (19) for receiving the composited light beam from
the optical composition member, whereby a relative displacement between
the first reflection member (28) and the second reflection member (14) is
detected based on a photoelectric conversion signal from the photoelectric
conversion member (19), said apparatus further comprising an asymmetric
conversion member (12) for establishing the optical intensity distribution
of a cross-section of at least one beam selected from a group of light
beams including the main light beam (L) irradiated from said light source
(11), said first light beam (MR1, MR2) and said second light beam (MM1,
MM2), said asymmetric conversion member being arranged at least one
portion on at optical path of light beam between the light source (11) and
the photoelectric converter (19), said optical distribution being
asymmetric relative to a center of said optical intensity distribution,
and a reflection member (29) for double path into which the first light
beam and the second light beam reflected by said first reflection member
(28) and said second reflection member (14) are entered and which reflects
said entered first light beam and said second light beam in the same
directions as those in which the beams are entered, such that said
reflected beams are directed again via said first reflection member (28)
and said second reflection member (14) into said optical composition
member (26).
In this case, an example of the reflection member for double path is a
corner-cube.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view showing a stage device of an interferemetric
distance measuring apparatus according to the present invention in order
to show a principle thereof as well as the first embodiment thereof.
FIG. 2a is a sectional view taken along the line A--A of FIG. 2b; FIG. 2b
is a view showing an example of a construction of the beam expander shown
in FIG. 1; and FIG. 2c is a sectional view taken along the line B--B in
FIG. 2b.
FIG. 3 is a perspective view showing an interference measuring portion 13
in FIG. 1.
FIG. 4a is a view showing the state of a superposition of each beam at an
incident surface of a receiver 19 shown in FIG. 1; and FIG. 4b is a plan
view of a corner-cube 29 shown in FIG. 3.
FIGS. 5a and 5b are views showing how a laser beam with an elliptic
cross-sectional shape is restored to a laser beam with a circular
cross-sectional shape.
FIG. 6 is a perspective view showing a light source system for generating a
laser beam with an elliptic cross-sectional shape by using a laser diode.
FIG. 7 is 8 perspective view showing a stage device of the second
embodiment of the interferometer according to the present invention.
FIG. 8a is a view showing the state of a superposition of each beam at an
incident surface of a receiver 19 shown in FIG. 7; and FIG. 8b is a plan
view of a corner-cube 29 in the interference measuring portion 13 shown in
FIG. 7.
FIG. 9a is a view showing a basic construction of the third embodiment of
the interferometer according to the present invention; and FIG. 9bis a
similar view showing a modification of the third embodiment.
FIGS. 10a-10f are views showing the state of a superposition of each beam
at an incident surface of a receiver 50 in the third embodiment and
various modifications thereof.
FIG. 11a is a view showing an optical path, whereby a laser beam with an
asymmetrical close-sectional shape is obtained by using a pair of
cylindrical lenses; and FIG. 11b is a sectional view of the laser beam
taken along the line C--C in FIG. 11a.
FIG. 12 is a plan view showing a stage device provided with a prior laser
interferometer.
FIG. 13 is a plan view showing how the measurement beam is displaced in
FIG. 12.
FIG. 14 is a view showing how the reference beam and the measurement beam
are superposed in FIG. 13.
FIGS. 15a-15c are views showing corner-cubes used in the interferometer of
a prior double path system.
DESCRIPTION OF THE PRINCIPLE OF THE INVENTION
The principle of the present invention will be described with reference to
FIG. 1. As a coordinate system, Y-axis is taken in parallel to the second
light beam (MM1 etc.) entering the second reflection member (14) and an
orthogonal coordinate system within the two dimensional plane
perpendicular to the Y-axis is taken as X-axis and Z-axis. A movable
mirror (14) fixed to the Y-stage (17Y) which is movable in the Y-direction
is assumed as the second reflection member (14), which can detect an
amount of movement of the Y-stage (17Y).
In this case, the direction of the displacement of the second light beam
(MM1, etc.) when the movable mirror (14) is tilted is represented by a
protection of a normal vector of a reflecting surface of the movable
mirror onto another surface perpendicular to an optical axis of the second
light beam (MM1, etc.). Further, an actual amount of displacement of the
light beam is obtained by multiplying a proportional constant determined
from a distance between the movable mirror (14) and the photoelectric
converter (19) by an amount of displacement of the projection of its
normal vector.
In FIG. 1, for example, if it is assumed that the normal vector of the
movable mirror (14) in the predetermined state is taken as (b, 1, a) in
the (X, Y, Z) coordinate system, and the distance from the movable mirror
(14) to the photoelectric converter (19) of the interference light is
taken as T, the vector in the direction of displacement of the movable
mirror (14) is obtained as (b, 0, a). Consequently, the vector showing an
amount Of displacement of the light beam of the interference light at the
position of the photoelectric converter (19) is obtained as 2T (b, 0, a).
Herein, if each amount of rotation of the movable mirror (14) about the
X-axis, Y-axis and Z-axis, respectively, appears successively as a minute
angle d.theta., the normal vector is obtained as follows:
(b, cos d.THETA.-a sin d.theta., sin d.theta.+a cos d.theta.),
(a sin d.theta.+b cos d.theta., 1, a cos d.theta.-b sin d.theta.),
(sin d.theta.+b cos d.theta., cos d.theta.-b sin d.theta., a)
Further, if terms of two or more dimensions are abbreviated, they can be
alternatively expressed as follows:
(b, 1-ad.theta., a+d.theta.),
(b+ad.theta., 1, a-bd.theta.),
(b+d.theta., 1-bd.theta., a)
Consequently, amounts of displacement of the light beam at the
photoelectric converter (19) are obtained as follows, where it is under a
single path system:
2T (0, 0, d.theta.),
2T (ad.theta., 0, -bd.theta.),
2T (d.theta., 0, 0)
Further, an amount of displacement of the second light beam (MM1, etc.)
which appears or is caused when the movable mirror (14) is tilted in any
direction can be obtained as a sum of these vectors.
The movable mirror (14) is in principle perpendicular to the entering
second light beam (measurement beam) (MM1). Therefore, if the tilting
angle of the movable mirror is minute, the condition of a <<1 and b <<1
can be assumed. Therefore, an amount of displacement of the light beam
caused by the rotation of the movable mirror (14) about the Y-axis (in
parallel to the second light beam (MM1, etc.)) is extremely small compared
to an amount of displacement caused by the rotation of the mirror (14)
about axes (X-axis, Z-axis) perpendicular to the second light beam (MM1,
etc.).
In the interferometer, as shown in FIG. 1, if a pitching or rolling
adjustment, e.g., is performed for the table (15) on which the movable
mirror (14) is mounted, the movable mirror (14) can possibly be rotated
about the X-axis or Y-axis, but not about the Z-axis. On the other hand,
if a yawing adjustment is performed, only the rotation of the mirror (14)
about the Z-axis is generated.
Relating to this, the present invention concerns with the case that only
either one of pitching or rolling adjustment or yawing adjustment is
performed for the table (15) to which the movable mirror (14) is fixed. If
only a pitching or rolling adjustment is performed, for example, the
movable mirror (14) rotates only about the X-axis or Y-axis, so that it
results in that the second light beam (MM1, etc.) is largely displaced
only in the Z-axis. On the contrary, if only a yawing adjustment is
performed, the movable mirror (14) is rotated only about the z-axis, with
the result that the second light beam (MM1, etc.) is largely displaced
only in the X-direction.
In the present invention ,therefore, if the second light beam (measurement
beam) (MM1, etc.) is displaced only in the Z-axis, a cross-sectional shape
of the light beam (L) from the light source (11) is expanded in the
Z-direction like a light beam (M), as shown in FIG. 2, which has an
asymmetric shape. Therefore, undesirable effects caused by the
displacement of the movable mirror can be reduced to a greater extent than
in the prior arts even when the second light beam is displaced in the
Z-direction by the same amount as that in the prior arts. On the other
hand, if the second light beam (MM1, etc.) is largely displaced only in
the X-axis, the light beam (L) from the light source (11) is expanded in
the X-direction, as shown in FIG. 7, which has an asymmetric shape.
Therefore, the undesirable effects caused by the displacement of the
movable mirror can be reduced to a greater extent than in the prior arts
even when the second light beam is displaced in the X-direction by the
same amount as that in the prior art.
Furthermore, if anamorphic prisms (24, 25) are used as an asymmetric
converters as shown in FIG. 2, it is possible to expand a circular
cross-sectional shape of the light beam (L) from the light source (11) in
the predetermined direction (Z-direction), into an asymmetric
cross-sectional shape of the light beam (L), without aberration.
Furthermore, if a pair of cylindrical lenses are used as asymmetric
conversion members, It is possible to expand or contract the
cross-sectional shape of the light beam (L) from the light source (11) in
the predetermined direction on the same optical axis as that at the time
of incidence of the beam.
Furthermore, if the asymmetric conversion member is arranged between the
light source (11) and the beam splitting member (26), the optical
intensity distribution in the cross-section of each of the first and the
second light beams, etc. is made asymmetric after the conversion member.
Furthermore, if an asymmetric light source (40, 41) like a laser diode,
e.g., is used, which generates a main light beam having asymmetric optical
intensity distribution, the source (40, 41) also serves as an asymmetric
conversion member so that the optical intensity distribution in the
cross-section of light beam on all the optical paths from the asymmetric
light source to the photoelectric converter (19) is made asymmetric.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The first embodiment of an interferemetric distance measuring apparatus,
so-called an interferometer according to the present invention will be
described with reference to FIGS. 1-4b in the following. This embodiment
is a double path type laser interferometer for coordinate measurement,
which is mounted On the stage device provided with a leveling mechanism
(tilting mechanism).
FIG. 1 shows a stage device provided with a laser interferometer according
to-the present invention. In FIG. 1, a laser beam L with a circular
cross-sectional shape irradiated from a laser light source 11 such as a
He--Ne laser light source passes through an asymmetric beam expander 12
whereby a cross-sectional shape of the beam L is changed into an ellipse,
a longer axis of which is in the direction parallel to the Z-axis. The
beam L then enters an interference measuring portion 13 for Y-axis as a
laser beam M. A measurement beam MMI (P-polarized light) emitted from the
interference measuring portion 13 enters an L-shaped movable mirror 14 in
the direction parallel to the Y-axis perpendicular to the Z-axis. The
X-axis is taken perpendicularly to a plane formed by the Z-axis and the
Y-axis.
The movable mirror 14 is provided with a reflecting surface substantially
perpendicular to the Y-axis and another reflecting surface substantially
perpendicular to the X-axis, wherein the measurement beam MM1 enters the
reflecting surface substantially perpendicular to the Y-axis. The
measurement beam MM1 reflected by the movable mirror 14 returns to the
interference measuring portion 13. The beam is then and further reflected
by the interference measuring portion 13 and enters the movable mirror 14
as a measurement beam MM2 in the direction substantially parallel to the
Y-axis. Then, the measurement beam MM2 reflected by the movable mirror 14
returns to the interference measuring portion 13. This measurement beam
MM2 enters, together with a reference beam MR2 (S-polarized light)
Generated in the portion 13, a receiver 19 comprising an analyzer and a
photoelectric conversion element, as an interference light in the
direction substantially parallel to the Y-axis.
The movable mirror 14 is fixed onto a mount table 15 for mounting a wafer
etc. This mount table 15, on the other hand, is mounted on an X-stage 17X
via a leveling mechanism (tilting mechanism) consisting of three
supporting points 16A-16C which are expandable in the Z-direction,
respectively. The X-stage 17X is mounted on a Y-stage 17Y so as to be
movable along the X-axis. The Y-stage 17Y, on the other hand, is mounted
on a base 18 so as to be movable along the Y-axis. An amount of movement
of the mount table 15 in the Y-direction can be measured by the laser
light source 11, the beam expander 12, the interference measuring portion
13, the movable mirror 14 and the receiver 19.
In a similar way, a laser beam from a laser light source 20 for X-axis is
expanded in the Z-direction by the beam expander 21, and enters an
interference measuring portion 22 for X-axis. The measurement beam emitted
from the interference measuring portion 22 is directed to a reflecting
surface of the movable mirror 14 substantially perpendicular to the
X-axis, and the measurement beam reflected by this reflecting surface
returns to the interference measuring portion 22. Then, the measurement
beam reflected by the interference measuring portion 22 is directed again
to the movable mirror 14.
The measurement beam reflected again by the movable mirror 14 is composited
with a reference beam inside the interference measuring portion 22 and
enters a receiver 23. An amount of movement of the mount table 15 in the
X-direction can be measured by the laser light source 20, the beam
expander 21, the interference measuring portion 22, the movable mirror 14
and the receiver 23.
FIG. 2b shows an example of a construction of the asymmetric beam expander
12 shown in FIG. 1. In FIG. 2b the cross-sectional shape of the laser beam
L irradiated from the laser light source 11 in FIG. 1 is a circular shape
with the diameter d as shown in FIG. 2a, The laser beam L then enters a
first anamorphic prism 24 with an incident angle .theta..sub.in, said
prism 24 having a refractive index n and a vertical angle 6. The incident
angle .theta..sub.in is selected such that the laser beam emitted from the
prism 24 is made perpendicular to a surface of the prism That is, the
following relation is obtained:
sin .theta..sub.sin =n . sin .delta.
The laser beam emitted from the first anamorphic prism 24 enters a second
anamorphic prism 25 with an incident angle .theta..sub.in, said prism 25
having a refractive index n and a vertical angle .delta.. From this prism
25, a laser beam M is emitted perpendicularly to a surface of the prism
25. In this case, the width D of the emitted laser beam M in the
Z-direction is expressed by using the width d of the entering laser beam L
in the Z-direction:
(D/cos .delta.) cos .theta..sub.in =(d/cos .theta..sub.in) cos .delta.
Therefore, the following formula is obtained:
D=(cos .delta./cos .theta..sub.in).sup.2 . d
In this embodiment, it is assumed that the condition is
.delta.<.theta..sub.in and D>d. For example, it is assumed that the width
D is equal to 2d. That is, the laser beam M having an elliptic
cross-sectional shape with the width D in the Z-direction is first
obtained, as shown in FIG. 2c, by expanding the cross-sectional shape of
the entering laser beam L in the Z-direction by the beam expander 12, and
this laser beam M enters the interference measuring portion 13 shown in
FIG. 1. In case that a pair of anamorphic prisms 24, 25 are used, it is
possible to obtain good asymmetric cross-sectional shape of the beam,
without a distortion of waveform in the laser beam.
In this embodiment, while the incident angle .theta..sub.in is selected
such that an exit angle of the laser beam emitted from the anamorphic
prism 24 is made 90.degree., the exit angle can take any angles other than
90.degree.. While a laser beam M with the length D in the Z-direction as
well as the length d in the X-direction is obtained by expanding in the
Z-direction the laser beam L with the diameter d, it is alternatively
possible to generate a laser beam with the diameter D from the laser light
source 11 and then to contract the laser beam in the X-direction by the
beam expander 12, thereby obtaining a laser beam M with the length D in
the Z-direction and the length d in the X-direction.
FIG. 3 shows a construction of the interference measuring portion 13 of
double path system shown in FIG. 1. In FIG. 3, within the laser beam M
with an elliptic cross-sectional shape entering the portion 13, two
polarized light components which are orthogonal to each other are
included. S-polarized light component polarized in the direction
perpendicular to an incident surface is reflected by a polarized beam
splitter (PBS) 26 and directed as a reference beam MR1 to a fixed mirror
28 through a 1/4-wave plate 27R. The reference beam MR1 reflected by the
limed mirror 28 passes through the 1/4-wave plate 27R and is transmitted
as P-polarized light through the polarized beam splitter 26, and is
further directed to a corner-cube 29 as P-polarized light. The reference
beam MR1, after reflected by the corner-cube 29 as a reference beam MR2
and transmitted through the polarized beam splitter 26, and enters,
through the I/4-wave plate 27, the fixed mirror 28. The reference beam MR2
reflected again by the fixed mirror 28 passes through the 1/4-wave plate
27R, is reflected by the polarized beam splitter 26, and then enters, as
S-polarized light, the receiver 19 shown in FIG. 1 in the direction
substantially parallel to the Y-axis.
On the other hand, P-polarized light component polarized in the direction
parallel to the incident surface of the polarized beam splitter 26 is
transmitted through the polarized beam splitter 26 and directed as a
measurement beam MM1 through the 1/4-wave plate 27M to the movable mirror
14. The measurement beam MM1 reflected by the movable mirror 14 passes
through the 1/4-wave plate 27M and is reflected by the polarized beam
splitter 26 and is directed to the corner-cube 29 as S-polarized light.
The measurement beam MM1 reflected by the corner-cube 29 returns as a
measurement beam MM2 to the polarized beam splitter 26. The measurement
beam MM2 reflected by the polarized beam splitter 26 is directed through
the 1/4-wave plate 27M to the movable mirror 14, and the measurement beam
MM2 reflected by the movable mirror 14 passes through the 1/4-wave plate
27M. This measurement beam MM2 is further transmitted as a P-polarized
light through the polarized beam splitter 26 and enters, as P-polarized
light, the receiver is shown in FIG. 1 in the direction substantially
parallel to the reference beam MR2.
In the receiver 19 in FIG. 1, a photoelectric conversion signal of the
interference light composited by the reference light MR2 and the
measurement light MM2 is generated. Based on this photoelectric conversion
signal, an amount of movement of the movable mirror 14 in the Y-direction
can be measured. In the case that the predetermined frequency difference
is given previously between the reference beam MR1 and the measurement
beam MM1, a measuring operation using the heterodyne interference method
is performed, and in the case that their beam frequencies are equal, on
the other hand, a measuring operation using the homodyne interference
method is performed. Furthermore, in the present embodiment of double path
system, the measurement beam travels reciprocally twice as MM1 and MM2
between the polarized beam splitter 26 and the movable mirror 14, thereby
the resolving power of measurement being improved to 1/2, as compared to
the embodiment of single path system.
In FIG. 3, the cross-sectional shape of the reference beam MR1, MR2
entering the fixed mirror 28 is an elliptic shape, a longer axis of which
is in the Y-direction, and the cross-sectional shape of The measurement
beam MM1, MM2 entering the movable mirror 14 is an elliptic shape, a
longer axis of which is in the Z-direction.
FIG. 4b is a plan view of the corner-cube 29 shown in FIG. 3 with the
diameter .phi.1. The reference beam MR1 and the measurement beam MM1 with
an elliptic cross-sectional shape, longer axis of which are in the
Y-direction, respectively, enter the corner-cube 29, and the reference
beam MR2 and the measurement beam MM2 with an elliptic cross-sectional
shape, longer axes of which are in the Y-direction in the same way,
respectively, are emitted therefrom. The beam space between the reference
beam MR1 and the measurement beam MR2 is W3, for example.
Operation of the present embodiment will be described. In an interference
optical system as shown in FIG. 3, when the movable mirror 14 is tilted
about axes perpendicular to the measurement beam MM1, MM2, i.e., X-axis or
Y-axis by the predetermined angle .theta., each of the measurement beam
MM1, MM2 is displaced in the direction parallel to the Z-direction or
X-direction, respectively. This amount of displacement is approximately
equal to 4T.theta. if the distance from a reflecting surface of the
movable mirror 14 to an apex of the corner-cube 29 is taken as T.
Relating to this, the mount table 15 according to the present embodiment is
mounted on the X-stage 17X via a leveling mechanism 16A-16C, as shown in
FIG. 1, so that, in FIG. 3, it is enough to take into consideration only
the rotational angle .theta..sub.x about an axis parallel to the X-axis in
various rotational angles of the movable mirror 14. Compared with this,
another rotational angle about the Z-axis is substantially negligible, and
there is little effect caused by the rotation about the Y-axis. That is to
say, when the measurement beams MM1 and MM2 are reflected by the movable
mirror 4, the direction of displacement of the beam due to the tilting of
the movable mirror 14 is mainly in the Z-direction and that an amount of
displacement of the beam in the other directions is negligible.
FIG. 4a shows the state of superposition of the reference beam MR2 and the
measurement beam MM2 at a light-receiving surface of the receiver is shown
in FIG. 1. As shown in FIG. 4a, in the present embodiment, the measurement
beam MM2 is displaced in the Z-direction by the distance .DELTA. relative
to the reference beam MR2 due to the tilting of the movable mirror 14.
However, both the reference beam MR2 and the measurement beam MM2 have an
elliptic cross-sectional shape, respectively, in which the length D in the
Z-direction is longer than the length d in the X-direction, so that
undesirable effects caused by an amount of displacement .DELTA. in the
Z-direction are suppressed as compared to the prior art.
Similarly, in the laser interferometer for X-axis shown in FIG. 1 including
the interference measuring portion 22, the cross-sectional shape of the
laser beam irradiated from the laser light source 20 is expanded in the
Z-direction. Furthermore, it is enough with regard to the reflecting
surface of the movable mirror 14 substantially perpendicular to the X-axis
to take into consideration only the rotational angle about the axis
parallel to the Y-axis, so that the direction of the displacement of the
measurement beam reflected by the movable mirror 14 is mainly in the
Z-direction. Therefore, in this laser interferometer for the X-axis as
well, undesirable effects caused by the tilting of the movable mirror 14
are suppressed or reduced by expanding the cross-sectional shape of the
laser beam in the Z-direction.
Now, the corner-cube 29 in FIG. 3 will be described in detail. While the
corner-cube in FIG. 2 is shown as a pyramid type for convenience of
explanation, it comprises actually a circular outline portion and a
pyramid portion at the backside thereof, as shown in FIG. 4b. In the
corner-cube 29 of the present embodiment, as shown in FIG. 4b, the
reference beam MR1 and the measurement beam MM1, etc. are aligned such
that their longitudinal directions (i.e. longer axes) are directed in the
direction parallel to one ridge line (a connection line between two
reflecting surfaces adjoining perpendicularly to each other) which is
parallel to the Y-direction. Furthermore, the reference beam MR1 and the
measurement beam MM1 etc., do not strike the other ridge lines. In order
to prevent each beam from striking the other ridge lines of the
corner-cube 29 as shown in FIG. 4b, it is required for the beam space W3
to satisfy the condition of the following formula:
W3.gtoreq.(3D.sup.2 +d.sup.2).sup.1/2
If it is assumed that for each beam, in FIG. 4a, the width d in the
X-direction is 4 mm and the width D in the Z-direction is 8 mm, it is
required in FIG. 4b, from the above formula, that the space W3 for the
double path should be 14.5 mm or more and that an effective diameter
.phi.1 of the corner-cube 29 should be 18.5 mm or more.
On the other hand, if a reference beam and a measurement beam of the same
power with a circular cross-sectional shape are used, it is required that
a beam diameter should be 8 mm in order to obtain the same rate of
superposed area as the above when the same amount of displacement as the
above occurs. In addition, as shown in the prior apt in FIG. 15c, it is
required to take 16 mm or more for the space W2 for the double path and to
take 54 mm or more for the effective diameter .phi.2 of the corner-cube 9.
It is also understood that the other optical parts may possibly be larger
in the prior art. Therefore, it is understood that, according To the
present embodiment, the size of the optical parts can be made smaller.
While, in the above embodiment, the laser beam converted so as to have an
elliptic cross-sectional shape by the beam expander 12 enters the receiver
19, the receiver 19 should have a light receiving area capable of
receiving a whole of the elliptic cross-section laser beam, which may make
the size of the receiver is slightly larger. Therefore, in order to avoid
this, i.e., to make the receiver 19 smaller, the cross-sectional shape of
the laser beam can be restored to the circular cross-sectional shape
before entering the receiver 19. In this case where the elliptic section
measurement beam and the elliptic section reference beam displaced to each
other are converted again into a laser beam with a circular
cross-sectional shape will be described with reference to FIGS. 5a and 5b.
FIG. 5a shows a reference beam OR1 and a measurement beam OM1 displaced to
each other by the distance .DELTA. in the Z-direction. In FIG. 5a, the
reference beam OR1 and the measurement beam OM1 have an elliptic
cross-sectional shape with the length d in the X-direction and the length
D in the Y-direction, respectively. When these two laser beams enter the
anamorphic prisms 24, 25 shown in FIG. 2b in the reverse direction, i.e.,
from the exit side of FIG. 2b, a reference beam OR2 and a measurement beam
OM2, each with a circular cross-sectional shape of the diameter d, can be
obtained, as shown in FIG. 5b. An amount of displacement between the
reference beam OR2 end the measurement beam OM2 in the Z-direction is also
changed into d.DELTA./D through the reduction rate of d/D which is the
same as that of the laser beam diameter. In addition, there is no
reduction of the interference signal intensity. Therefore, it is possible
to use a receiver with a smaller effective diameter corresponding to the
diameter d of the laser beam in place of the receiver 19 in FIG. 1.
Furthermore, in an interferometer in which the beam entering the
interference measuring portion 13 and the beam emitted from the same are
directed in parallel and in reverse direction to each other, as shown in
FIGS. 1 and 3, it is possible without the addition of a new optical system
to make an elliptic section laser beam to be restored to a circular
section laser beam by arranging a beam expander 12 at the position where
both beams pass.
In a similar way, in place of the beam expander 12 positioned before the
interference measuring portion 13 in FIG. 1, a beam reforming optical
system comprising anamorphic prisms or cylindrical lenses, etc. can be
arranged between the interference measuring portion 13 and the movable
mirror 14, e.g., between the 1/4-wave plate 27M and the movable mirror 14.
In this case, the cross-sectional shape of the laser beam is elliptic
between the beam reforming optical system and the movable mirror 14.
Therefore, if it is assumed that the magnification in the direction of a
longer axis of the ellipse established by this beam reforming optical
system is .alpha., an amount of displacement of the laser beam caused by
the movable mirror 14 is reduced into 1/.alpha., when the elliptic section
beam is restored to the circular section beam after a reciprocal travel
through the beam reforming optical system. Therefore, in this case as
well, undesirable effects caused by the rotation of the movable mirror 14
about the X-axis, which may occur between the beam reforming optical
system and the movable mirror 14, can be reduced in the way similar to the
above-mentioned embodiment.
In the above-mentioned embodiment, asymmetric beam expanders 12, 21 are
used in order to obtain a laser beam with an asymmetric optical intensity
distribution in its cross-section. In place of this, however, a laser
light source in which the optical intensity distribution of the radiated
laser beam is asymmetric can be used.
FIG. 6 shows an embodiment of such an asymmetric laser light source. In
FIG. 6, a laser beam of linearly polarized light is irradiated from a
laser diode 40 on condition that a divergent angle of the beam is greater
in the Z-direction than that in the X-direction. Then, a laser beam M2
with an elliptic cross-sectional shape, a longer axis of which is in the
Z-direction, is obtained by converting the laser beam irradiated from the
laser diode 40 into a parallel luminous flux by using the collimator lens
41. Meanwhile, the laser beam M2 is the linearly polarized light polarized
in a shorter axis direction (X-direction), so that the linearly polarized
light of the laser beam M2 is further converted into circularly polarized
light when it passes through the 1/4-wave plate 43. Thereafter, when this
laser beam M2 enters the polarized beam splitter 26 in FIG. 3, e.g., an
interferometer of the homodyne interference method is achieved. In this
case, a laser beam with an elliptic cross-sectional shape can be obtained
without the need of another asymmetric beam expander, thereby enabling the
interferometer to be made smaller.
Now, the second embodiment of the present invention will be described with
reference to FIGS. 7, 8a and 8b. In FIGS. 7, 8a and 8b, parts
corresponding to FIGS. 1-3 are given the same reference numerals.
FIG. 7 shows a stage device of a laser interferometer according to the
second embodiment. In FIG. 7, a laser beam L with a circular
cross-sectional share irradiated from a laser light source 11 passes
through an asymmetric beam expander 12A as a laser beam N with an elliptic
cross-sectional shape, a longer axis of which is in the X-direction, and
thereafter enters an interference measuring portion 13 for Y-axis. A
measurement beam NM1 emitted from the interference measuring portion 13
enters in parallel to the Y-axis, an L-shaped movable mirror 14. The
measurement beam NM1 reflected by the movable mirror 14 returns to the
interference measuring portion 13. The measurement beam NM2 further
reflected by the interference measuring portion 13 enters, substantially
in parallel to the Y-axis, the movable mirror 14. Then, the measurement
beam NM2 reflected by the movable mirror 14 returns again to the
interference measuring portion 13. This measurement beam NM2 enters,
together with a reference beam NR2 generated in the portion 13, a receiver
19, as an interference light in the direction substantially parallel to
the Y-axis.
The movable mirror 14 is fixed onto a mount table 15. The mount table 15 is
mounted on an x-stage 17X via a rotary table 30 rotatable about an axis
parallel to the Z-axis. Furthermore, a laser beam from a laser light
source 20 for X-axis is expanded in the Y-direction by the beam expander
21A, and enters an interference measuring portion 22 for x-axis. The
measurement beam emitted from the interference measuring portion 22 is
directed to a reflecting surface of the movable mirror 14 substantially
perpendicular to the X-axis, and the measurement beam reflected by this
reflecting surface returns to the interference measuring portion 22. Then,
the measurement beam reflected by the interference measuring portion 22 is
directed again to the movable mirror 14.
The measurement beam reflected again by the movable mirror 14 is composited
with a reference beam inside the interference measuring portion 22, and
enters a receiver 23. The other constructions are the same as those in
FIG. 1.
In the above second embodiment, it is enough to take into consideration
only the rotational angle .theta..sub.z about an axis parallel to the
Z-axis in various rotational angles of the movable mirror 14, so that the
other rotational angles are negligible. That is, it can be assumed that
the measurement beams NM1 and NM2 emitted from the interference measuring
portion 13 and entering the movable mirror 14 are displaced only in the
X-direction by the rotation of the movable mirror 14. In a similar way, it
can be assumed that the measurement beam emitted from the interference
measuring portion 22 and entering the movable mirror 14 is displaced only
in the Y-direction by the rotation of the movable mirror 14.
FIG. 8a shows the state of superposition of the reference beam NR2 and the
measurement beam NM2 on a light-receiving plane of the receiver 19 in FIG.
1. As shown in FIG. 8a, in the present embodiment, the measurement beam
NM2 is displaced in the X-direction relative to the reference beam NR2 due
to the tilting of the movable mirror 14. However, both the reference beam
NR2 and the measurement beam NM2 have an elliptic cross-sectional shape,
respectively, an axis in the X-direction is longer than that in the
Z-direction, so that undesirable effects caused by the displacement in the
X-direction is suppressed as compared to the prior art.
In the same way, in the laser interferometer for the X-axis in FIG. 7
including the interference measuring portion 22, the cross-sectional shape
of the laser beam irradiated from the laser light source 20 is expanded in
the Y-direction. Furthermore, it is enough for a reflecting surface of the
movable mirror 14 substantially perpendicular to the X-axis to take into
consideration only the rotational angle about the axis parallel to the
Z-axis, so that the direction of the displacement of the measurement beam
reflected by the movable mirror 14 is mainly in the Y-direction.
Therefore, in this laser interferometer for the X-axis as well,
undesirable effects caused by the tilting of the movable mirror 14 can be
reduced by expanding the cross-sectional shape of the laser beam in the
Y-direction.
Now, the corner-cube 29 in the interference measuring portion 13 in FIG. 7
will be described in detail. In the corner-cube 29 of the second
embodiment, as shown in FIG. 8b, the reference beam NR1 and the
measurement beam NM1 in the first path and the reference beam NR2 and the
measurement beam NM2 in the second path are aligned such that their
longitudinal directions (i.e. longer axes) are directed in the direction
perpendicularly to one ridge line (a connection line between two
reflecting surfaces adjoining perpendicularly to each other) which is
parallel to the Y-direction. Furthermore, the reference beam NR1 and the
measurement beam NM1, etc. do not strike the other ridge lines.
If it is assumed that for each beam NR2, NM2, in FIG. 8a, the width in the
Z-direction is 4 mm and the width in the X-direction is 8 mm, it is
required in FIG. 8b, that the space W4 for the double path should be 10.6
mm or more and that an effective diameter .phi.1 of the corner-cube 29
should be 18.6 mm or more.
On the other hand, if a reference beam and a measurement beam of the same
power with a circular cross-sectional shape are used, it is required that
a beam diameter should be 8 mm in order to obtain the same rate of
superposed area as the above when the same amount of displacement as the
above occurs. In addition, as shown in the prior art in FIG. 15c, it is
required to take 16 mm or more for the space W2 for the double path and to
take 24 mm or more for the effective diameter .phi.2 of the corner-cube 9.
It is also understood that the other optical parts may possibly be larger
in the prior art. Therefore, it is understood that, according to the
present embodiment, the optical parts can be made smaller.
Now, the third embodiment of the present invention will be described with
reference to FIGS. 9a, 9b and 10a-10e.
FIG. 9a shows a laser interferometer of the third embodiment. In FIG. 9a, a
laser beam with a circular cross-sectional shape of the diameter d
irradiated from a laser source 45 is reformed by a beam reforming unit 46A
into a laser beam with an elliptic cross-sectional shape having the width
D in the X-direction and the width d (d<D) in the Z-direction (the
direction Z is perpendicular to a sheet surface in FIG. 9a). The laser
beam emitted from the beam reforming unit 46A is divided into a reference
beam and a measurement beam by a beam splitter 47, The reference beam
reflected by the beam splitter 47 is made to return along the optical path
by a corner-cube 48, and is reflected again by the beam splitter 47. This
beam finally enters a receiver 50 as a reference beam PR1.
On the other hand, the measurement beam transmitted through the beam
splitter 47 is made to return along the optical path by the corner-cube
49, is transmitted again through the beam splitter 47. This beam finally
enters the receiver 50 as a measurement beam PM1. At that time, the
measurement beam PM1 is displaced in the X-direction by the distance which
is twice of an amount of displacement of the apex of the corner-cube 49 in
the X-direction. That is, when the corner-cube 49 is displaced by the
distance .DELTA./2 in the X-direction into the position 49A, the
measurement beam PM1 is displaced by the distance .DELTA. on the receiver
50. It is possible to detect an amount of the displacement of the
corner-cube 49 in the Y-direction by photoelectrically converting the
interference light of the reference beam PR1 and the measurement beam PM1
at the receiver 50. At that time, the reference beam PR1 and the
measurement beam PM1 are displaced each other by the distance .DELTA. in
the X-direction, as shown in FIG. 10a. On the other hand, in the case that
there is no beam reforming unit 46A in FIG. 9a, the reference beam PR1 and
the measurement beam PM1 detected at the receiver 50 are shown In FIG.
10f. It is understood from the comparison between FIGS. 10a and 10f that,
in the third embodiment, the S/N ratio of the interference signal obtained
from the receiver 50 can be improved by using the beam reforming unit 46A.
Furthermore, as shown in FIG. 9b, one Or more of beam reforming units
46B-46D can be arranged at the positions which are different from that of
the beam reforming unit 46A in FIG. 9a. In FIG. 9b, parts corresponding to
FIG. 9a are given the same reference numerals. The beam reforming unit 46B
is arranged on a going path from the beam splitter 47 to the corner-cube
49, the beam reforming unit 46C is arranged on a returning path from the
corner-cube 49 to the beam splitter 47, and the beam reforming unit 46D is
arranged on a returning path from the corner-cube 48 to the beam splitter
47, respectively. It is also possible to use any one of the beam reforming
units 46B-46D or use any two of them in combination.
In this case, the beam reforming units 46B, 46D mounted so as to expand the
laser beam by the magnification ratio D/d along the travel direction of
the laser beam as well as in the direction parallel to a sheer surface in
FIG. 9b, which is the same as the case of the beam reforming unit 46A. On
the contrary, the beam reforming unit 46C is mounted so as to contract the
laser beam by the contraction ratio d/D along the travel direction of the
laser beam as well as in the direction parallel to the sheet surface of
FIG. 9b. That is, the beam reforming units 46B and 46D are mounted in the
reverse direction to each other along the travel direction of the laser
beam. As each beam reforming units 46A-46D, a pair of anamorphic prisms or
a pair of cylindrical lenses, etc. can be used. Furthermore, if both of
the beam reforming units 46B and 46C are used, they can be made into one
part by arranging a pair of anamorphic prisms across both the going and
returning paths at the position between the beam splitter 47 and the
corner-cube 49.
Cross-sectional shapes and amounts of displacement of the laser beams
obtained on the receiver 50 on condition that the beam reforming units
46B-46D are combined in various ways will be described with reference to
FIGS. 10b-10e. In every figure, the reference beam is depicted by a
reference beam PR2 of solid lines and the measurement beams is depicted by
a measurement beam PM2 of dotted lines.
FIG. 10b shows a case in which only one beam reforming unit 46B is used. On
an incident surface of the receiver 50, the measurement beam PM2 with an
elliptic cross-sectional shape having the width D in the X-direction and
the width d in the Z-direction is displaced by the distance .DELTA. in the
X-direction from the reference beam PR2 with a circular cross-sectional
shape. FIG. 10c shows a case in which only one beam reforming unit 46C is
used. On the incident surface of the receiver 50, the measurement beam PM2
with an elliptic cross-sectional shape having the width d.sup.2 /D in the
X-direction and the width d in the Z-direction is displaced by the
distance (d/D)..DELTA. in the X-direction from the reference beam PR2 with
a circular cross-sectional shape. FIG. 10d shows a case in which only one
beam reforming unit 46D is used. On the incident surface of the receiver
50, the measurement beam PM2 with a circular cross-sectional shape having
the diameter d is displaced by the distance .DELTA. in the X-direction
from the reference beam PR2 with an elliptic cross-sectional shape having
the width D in the X-direction and the width d in the Z-direction.
If any one of these beam reforming units 46B, 46C, 46D is used, an
interference signal substantially similar to the case shown in FIG. 10f
can be obtained, in said case in FIG. 10f no beam reforming unit being
used. However, in the interferometer of the homodyne method, e.g., if the
laser beam is split by the beam splitter 47 such that both amounts of
light per unit area of the reference beam PR2 and the measurement beam PM2
on the incident surface of the receiver 50 in FIG. 9b are substantially
equal to each other, a stronger interference signal can be obtained.
FIG. 10e shows a case in which both the beam reforming units 46B and 46C
are used. On the incident surface of the receiver 50, the measurement beam
PM2 with a circular cross-sectional shape having the diameter d is
displaced by the distance (d/D)..DELTA. in the X-direction from the
reference beam PR2 with a circular cross-sectional shape having the
diameter d. In this case, an interference signal similar to that in the
case shown in FIGS. 9a and 10a can be used, in which FIGS. 9a and 10a the
beam reforming unit 46A is used. Furthermore, if both the beam reforming
units 46B and 46D are used, two laser beams obtained on the receiver 50
are like those shown in FIG. 10a. Therefore, it is obvious that it is
possible to obtain the same effects as those in the case shown in FIG. 9a
in which the beam reforming unit 46A is used.
Furthermore, the embodiments of FIGS. 9a and 9b refer to the case in which
the measurement beam is displaced in the X-direction. Meanwhile, if the
corner-cube 49 is displaced in the Z-direction, i.e., in the direction
perpendicular to the sheet surface of FIGS. 9a and 9b, for example, it is
possible to suppress undesirable effects caused by the displacement of the
corner-cube 49 in the Z-direction by rotating the beam reforming unit 46A,
46B, 46C or 46D about the corresponding optical axis by 90.degree..
Furthermore, when a plurality of beam reforming units are used, although,
in the above-mentioned embodiments, the beam reforming units with the same
magnifications are combined, a plurality of beam reforming units with
different magnifications can be combined.
Furthermore, in all the above-mentioned embodiments, all the laser beams
each with an asymmetric cross-section are elliptic cross-section laser
beams each with uniform optical intensity distribution in the
corresponding cross-section. However, without being limited to them, a
laser beam with a rectangular cross-sectional shape, a longer axis of
which is in the direction of the displacement or another laser beam even
with a circular cross-sectional shape, under the condition that the
optical intensity distribution in the circular cross-sectional shape is
asymmetric, can bring about the same effects as those in the
above-mentioned embodiments. The reason for this is that a reduction rate
of the interference intensity occurring when the beam is displaced in the
predetermined direction can be made smaller than that occurring when the
beam is displaced by the same distance in another direction.
Furthermore, in place of the anamorphic prisms, as shown in FIG. 11a, a
pair of cylindrical lenses 51A and 51B can be used in order to obtain a
laser beam with an asymmetric cross-sectional shape. In FIG. 11a, the
positive and negative cylindrical lenses 51A, 51B do not have refractive
power in the direction perpendicular to the sheet surface in FIG. 11a
(X-direction). Furthermore, focal depths of the cylindrical lenses 51A,
51B in a plane parallel to the sheet surface is taken as -f.sub.1, f.sub.2
(f.sub.1 <f.sub.2). Then, an entering laser beam L with a circular
cross-sectional shape is converted, at the time of emission, into laser
beam M, a cross-sectional shape of which is expanded in the Z-direction at
the rate of f.sub.2 /f.sub.1, as shown in 11b. When a pair of cylindrical
lenses 51A, 51B are used in such a way, as the two laser beams L and M are
aligned on one optical axis 52, an optical system can be arranged easily.
Furthermore, while a beam expander of Galileo type comprising concave and
convex cylindrical lenses 51A, 51B are used here, a beam expander of
Kepler type comprising two convex cylindrical lenses can be alternatively
used.
Furthermore, in place of anamorphic prisms or cylindrical lenses, a beam
expander for expanding or contracting the light beam in the predetermined
direction, or a slit plate having a long and narrow opening, or a
combination of both of them can be used. That is, any optical member which
can elongate or expand an area of the optical intensity distribution in
the direction of the displacement of the light beam can be used,
regardless of cross-sectional shapes of the beam.
Furthermore, as a reflection member for the double path system, in place of
the corner-cube, a combination of two mirrors or two prisms can be used on
condition that two reflecting surfaces thereof are substantially
perpendicular to each other.
The interferometer in accordance with each of the above-mentioned
embodiments is proper to be applied, for example, to an exposure apparatus
used in a photolithography process for producing semiconductor elements,
liquid crystal display elements or thin film magnetic heads, especially to
a mask stage for mounting thereon masks or to a substrate stage for
mounting thereon photosensitive substrates (for example, semiconductor
wafers or glass plates, etc.) onto which patterns on the mask are
transferred. As exposure apparatuses of such types, U.S. Pat. Nos.
5,214,489 and 5,243,195 disclose stepping and repeating type projection
exposure apparatuses (so-called, stepper) for reducing a pattern image
prepared on a mask via a projection optical system and for transferring a
reduced image onto the substrate. In addition, U.S. Pat. No. 5,194,893
discloses a stepping and scanning type projection exposure apparatus for
transferring a pattern image prepared on a mask onto a substrate by
synchronously scanning the mask and the substrate at a speed ratio
corresponding to the projection magnification of the projection optical
system. In addition, U.S. Pat. No. 5,298,761 discloses a projection
exposure apparatus for mounting a plurality of masks on a mask stage and
driving both of the mask stage and substrate stage so as to connect a
pattern images prepared on masks on the substrate, thereby, forming a
pattern, on the substrate, larger than the image field of a projection
optical system.
Furthermore, in the above-mentioned exposure apparatus, the fixed mirror 28
of the interferometer can be mounted on a mirror tube of the projection
optical system.
The present invention is not limited to the above-mentioned embodiments,
but can take other various constructions without deporting from the scope
of the present invention defined by appended claims.
According to the present invention, it is advantageous that an area the
optical intensity distribution of the cross-section of the light beam can
be expanded or contracted in the predetermined direction, so that it is
possible, without making the size of optical parts larger, to make the
permissible amount of displacement between the first light beam (reference
beam) and the second light beam (measuring beam) larger than that in the
prior art.
Furthermore, in the case that the asymmetric conversion member is
anamorphic prisms, there is hardly any undesirable effect exerted on the
wave plane aberration of the light beam when the cross-sectional shape of
the light beam is expanded or contracted by the anamorphic prisms, so that
good interference measurement can be conducted.
Furthermore, in the case that asymmetric conversion member is a pair of
cylindrical lenses, the entering beam and the emitted beam with a changed
cross-sectional shape by the lenses are aligned substantially on the same
axis, so that it may become easy to arrange a whole of optical system.
Furthermore, in the case that the asymmetric conversion member is arranged
between the light source and the beam splitting member, the optical
intensity distribution of the cross-section of all the light beams
including the first light beam and the second light beam An the downstream
of the asymmetric conversion member can be made asymmetric.
Furthermore, in the case that an asymmetric light source for generating the
main light beam is used as the light source on condition that the optical
intensity distribution of the cross-section is asymmetric, and that this
asymmetric light source also serves as an asymmetric conversion member, a
construction of the optical system can be simplified.
Furthermore, when the present invention is applied to an interferometer for
the double path system, it is possible, without making the size of optical
parts larger, to make the permissible amount of displacement between the
first light beam (reference beam) and the second light beam (measurement
beam) larger than that in the prior art.
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